This invention relates to an overcurrent control device whose function is to detect an overcurrent condition in a circuit carrying large amounts of direct current, and to protect the circuit against such a condition by causing an external interrupting device to open the circuit.
Several lines of approach to the task of overcurrent control have been employed in commercially available devices. An electromechanical relay capable of operation by a predetermined amount of current in the protected circuit constitutes the most direct approach, although it is difficult to make an adjustable device of this type capable of dependable operation at a sufficiently high level of current. For this reason and because numerous applications require the detection means to be adjustable from a location remote from the protected circuit to reduce the danger of electrical shock, more complex devices have been developed which allow a representative sample of the current in the protected circuit to be extracted and transmitted to the desired remote location, to control a detection means of conventional design and capable of dependable adjustment to a predetermined level. One such device employs a meter shunt or other series resistance in the protected circuit to develop a low-level voltage proportional to the current in the circuit, which voltage is then applied to a suitable detection circuit. Another device of prior art employs a saturable reactor which effectively transforms the cumbersome level of direct current in the protected circuit to a proportional and much lower alternating current for controlling the detection means. This approach also provides the desirable feature of complete electrical isolation of the detection means from the protected circuit, thus permitting sensitive semiconductor components to be used as desired without exposure to damage by high-voltage transients originating in the protected circuit. Devices such as these are characterized by a common detriment, in that the complexity added to meet the various needs of application affords numerous possibilities for component failure or broken connections whereby the intended function of circuit protection is lost without warning, thus creating a hazardous situation.